Hunting for planets around nearby stars
The UK ATC was invited to participate in the Royal Society's prestigious Summer Exhibition 2004. An introduction to the science behind the exhibit is given below. The posters that supported the exhibit are also available through the menu on the left.
Other solar systems hidden in Stardust
Artist’s impression of the Vega planetary system. The Neptune-like planet can be seen in the foreground with the Jupiter-like planet orbiting nearer the star.
The two clumps of dust are visible at 90º to the position of the Neptune-like planet. Credit: David A Hardy/UKATC
Astronomers at the UK Astronomy Technology Centre at the Royal Observatory, Edinburgh have produced compelling evidence of a planetary system which is more like our own Solar System than any other so far discovered. It is around Vega, one of the brightest stars in the sky.
'Neptune' is the key
The SCUBA image: This is a false colour image of the heat emitted from the dust disk around Vega.
The image shows the disk seen face-on. The disk structure includes two bright clumps, represented
by the yellow and red colours. The star is barely noticeable and is located at the centre of the
image, mid-way between the two clumps. The position of the star (*) and the predicted position and
direction of the planet’s obit (+) are marked.New computer modelling techniques have shown that observations of the structure of a faint dust disk around Vega can be best explained by a Neptune-like planet orbiting at a similar distance to Neptune in our own solar system. The wide orbit of the Neptune-like planet means that there is plenty of room inside it for small rocky planets similar to the Earth – the Holy Grail for astronomers wanting to know whether we are alone in the Universe.
The modelling is based on observations taken with the world's most sensitive submillimetre camera, SCUBA. The camera, built in Edinburgh, is operated on the James Clerk Maxwell Telescope in Hawaii. The SCUBA image shows a disk of very cold dust (-180ºC) in orbit around the star.
Clumps in the disk
Prediction: The modelling is able to predict the structure of the disk at a higher resolution than can actually be seen. This is what the disk would look like if observed with a telescope ten times bigger than the JCMT.
The irregular shape of the disk is the clue that it is likely to contain planets. Although it is not possible to directly observe the planets, they have created clumps in the dust disk around the star.
Similar history to our Solar System
The modelling suggests that the Neptune-like planet actually formed much closer to the star than it is now. As it moved out to its current wide orbit over about 56 million years, many comets were swept out with it, causing the dust disk to be clumpy.
Exactly the same process is thought to have happened in our Solar System. Neptune was forced away from the Sun because of the presence of Jupiter orbiting inside it. So it appears that as well as having a Neptune-like planet, Vega may also have a more massive planet in a smaller orbit rather like Jupiter.
Submillimetre Astronomy
The James Clerk Maxwell Telescope in Hawaii was used to take the image of the Vega dust disk. It is the world’s largest single-dish submillimetre telescope.
Credit: Nik Szymanek
The submillimetre waveband lies between the better known infrared and radio bands of the electromagnetic spectrum. It is the waveband that best shows very cold gas and dust. One of the most important discoveries of the last two decades was that many nearby stars are surrounded by cold dust disks that come from a reservoir of comets orbiting further from their stars than Neptune is from the Sun.
Finding planets around other stars is difficult
The light from a star is so bright that it swamps the light coming from any planets. This means that astronomers have to find clever ways to look for them.
- The gravity of a large planet causes its star to wobble. Astronomers can detect this wobble.
- Stars like the sun give out a lot of visible light and heat, but at other wavelengths they hardly give out anything. If you use a telescope sensitive to these wavelengths you can see the disk of comets, asteroids and dust which orbit the star. Structure such as clumps in these disks is providing the first evidence of relatively small Neptune-like planets orbiting in these systems.